1. Field of the Invention
This invention relates to a tape reel for a tape cassette, such as a VHS cassette or 8 mm video cassette, that holds within its housing a pair of reels around which a length of tape is wound.
2. Description of the Prior Art
Contrivances for making tape reels as light as possible, while reducing the distortion due to uneven curing of the resin at the time of molding and minimizing the sacrifice of strength, have been proposed in prior art documents, for example, by Japanese Patent Application Kokai No. 55-87352. FIGS. 7 and 8 in the accompanying drawings illustrate an embodiment of the invention described in the specification of the above application. A tape reel 55 is shown comprising a hub which consists of a central drum 62 formed in one piece with, or solidly joined to, a lower flange 72, and an upper flange 73 fixed to the hub, with a spring (8 in FIG. 1) pressing a center boss 54 at the top of the central drum 62 in place. The central drum 62 consists of an outer cylindrical part 71 on which a tape is wound and an inner cylindrical part 74 adapted to receive a reel drive shaft (not shown) of a device into which the cassette is to be loaded. The inner cylindrical part 74 has a hollow 79 formed open at the bottom and extending short of the top of the part, so that the driving shaft of the recording-reproducing device can enter the inner cylindrical part 74 to drive the tape reel 55. The inner cylindrical part 74 has a plurality of inward projections 77 adapted to mesh with the driving shaft. Each of the projections 77 has a groove 75 formed on the back side to reduce its thickness. The back grooves 75 must be limited in size lest they should cause inward deformation of the inner cylindrical part 74 under pressure from the tape wound thereon, which would decrease the diameter of the center hollow and hamper the insertion of the driving shaft from the recording-reproducing device. The outer cylindrical part 71 and the inner cylindrical part 74 are secured together with a plurality of connecting ribs 78.
If the inner cylindrical part 74, its inward projections 77 and side walls 76 were thus made as thin as possible while avoiding substantially unfavorable effect upon their strength, it would lead to uniform cooling of the resin after molding, reduction of molding strains, and shortening of the molding cycle.
However, the prior art technique described above is still unable to realize the shortening of the molding cycle satisfactorily. With the conventional design the inner ends of the projections 77 have to be thicker than the rest of the inner cylindrical part to cope with the speedup of the molding cycle. This naturally increases molding shrinkage which tends to cause malformation of the inner cylindrical part, but the problem has not been completely settled yet. If the inner cylindrical part is made as thin as the remainder by increasing the depth of the back grooves, there will be the danger of the bore of the inner cylindrical part 74 being deformed under the pressure, transmitted through the connecting ribs 78, of the tape when the cassette has been stored for long with the tape wound thickly on the particular inner cylindrical part. This is particularly true with an inner cylindrical part of a cassette left inserted in a recording-reproducing device whose internal temperature is higher than the ambient temperature at which the cassette is ordinarily stored. Once the inner cylindrical part has been deformed to such an extent that it does not receive the driving shaft of the device when the cassette is loaded into the device, recording or reproducing will no longer be possible.
In view of these, the present invention aims at providing a tape reel equipped with means to prevent the deformation of the inner cylindrical part when it is made up of walls of the same thickness and thereby accelerating the molding cycle and reducing the molding strains of tape reels.